TRANSACTIONS OF THE MICROSCOPICAL 
SOCIETY OF LONDON. 
A New Adjustable Diaphragm. By Sidney B. Kincaid, 
Esq., F.R.A.S. 
(Communicated by F. H. Weniiam, Esq.) 
(Read March 14tii, 1866.) 
The desirability of possessing a means of adjusting the 
illumination of transparent objects under tlie compound 
microscope within closer limits than those allowed by the 
ordinary wheel of diaphragms placed beneath the stage, has 
been patent to microscopists almost ever since the study of 
the more minute forms of nature under high magnifying 
power has claimed to rank as a science. But although 
practical opticians have proposed various aperture-limiting 
shutters for attaining that object, no contrivance, as far as 
I am aware, has hitherto been described which fulfils the 
condition of affording an easily adjustable aperture, which 
constantly preserves its centricity, and approximates more 
nearly to a circular figure than a square or diamond. 
When I turned my attention to the subject a short time 
since it appeared to me that the adaptation of the Iris 
diaphragm (which was designed some years ago to be ap- 
plied to the astronomical telescope for the purpose of observ- 
ing variable stars) beneath the stage of the microscope would 
at once furnish the wished-for desideratum ; and the experi- 
ment has proved so satisfactory that, in the hope that it may 
be of service to others engaged in microscopic pursuits, I 
would beg leave to lay a description of it before the Micro- 
scopical Society. 
The arrangement, of which a sectional view is shown in 
Fig. 1, consists of a brass tube, A, screwing beneath the stage 
of the microscope at B, and within which a second tube, C, of 
less length, works friction tight ; this latter is sprung and 
furnished at one end with a milled edge, D, projecting 
beyond the outer tube, and affording the means of rotating 
the inner one. To the opposite extremities of these is attached 
VOL. XIV. / 
